Strain multiplier

ABSTRACT

A strain multiplier is provided which is designed to be firmly attached to both a structural surface and a sensing gage at all points of contact between the multiplier and the structural surface and the multiplier and the sensing gage while preventing direct contact between the sensing gage and the structural surface.

United States Patent [1 1 Starr STRAIN MULTIPLIER [75] Inventor: JamesE. Starr, Plymouth, Mich.

Vishay lntertechnology, Inc., Malvern, Pa.

221 Filed: Apr. 29, 1971 211 Appl. No.: 138,522

[73] Assignee:

I II V Jan. 1, 1974 3,272,003 9/1966 Harting.,. 73/885 R X 3,351,88011/1967 Wilner 73/885 R X 3,447,117 5/1969 Duffield... 73/141 A X3,602,041 8/1971 Weinert 73/116 FOREIGN PATENTS OR APPLICATIONS 654,9817/1951 Great Britain 73/885 R 853,755 11/1960 Great Britain 73/885 RPrimary Examiner-Charles A. Ruehl Att0rneyThomas M. Ferrill, Jr. andRoger Norman Coe [57] ABSTRACT A strain multiplier is provided which isdesigned to be firmly attached to both a structural surface and asensing gage at all points of contact between the multiplier and thestructural surface and the multiplier and the sensing gage whilepreventing direct contact between the sensing gage and the structuralsurface.

3 Claims, 9 Drawing Figures PAIENIEUJAII 1:974 3.782.182

SIIEEIllIF 2 SENSING GAGE MULTIPLI-ER I MEASURING INSTRUMENT gfl'g ggfiPRIOR ART F 1g I SENSING GAGE MULTIPLIER PRIOR ART I ADHESIVE SENSINGGAGE MULTIPLIER STRUCTURAL SURFACE ADHESIVE PRIOR ART F 7g Z/b/ SENS'NGGAGE MULTIPLIER PRIOR ART F 7g 3(a) ADHESIVE MULTIPLIER SENSING GAGESTRUCTURAL ADHESIVE SURFACE sansinasn PRIOR ART INVENTOR. $9 Fl JAMES E.STARR ATTORNEY.

PAIENIED'JAH 1 I974 sum 20F 2 P F 30 25 7 ///,W

6 INVENTOR.

' JAMES E. STARR BY I g ATTORNEY.

STRAIN MULTIPLIER BACKGROUND OF THE INVENTION This invention relatesbroadly to the field of materials testing, and more particularly, thepresent invention relates to the field of materials testig whereinstructural strains, due to stress, are to be measured or sensed withstrain responsive devices, such as bonded resistance strain gages orfatigue life gages.

It is known that when strains in structural materials, caused by appliedloads or stresses, are sufficiently large they can be accurately sensedor measured by bonded metallic resistance strain gages. When suchstrains are caused by cyclic loading, which may lead to structuralfatigue failure, they can be monitored by bonded fatigue life gages in amanner described by U. S. Letters Patent No. 3,272,003. However, if theamplitude of structural strains is too low it becomes difficult orimpossible to use conventional strain gages or fatigue gages to monitoror measure such strains. The problem of measuring low strain levels withsuch gages is encountered, for example, when dealing with weakstructural materials, or when strains in one area of a structure must bemonitored by gages installed on another part of the structure whereproportional, but lower amplitude strain levels exist. Low strain levelsare commonly encountered with fatigue life gages since these gagesshould normally be mounted on the part of a structure which would firstfail under cyclic loading. These failure areas are often inaccessible,or hidden by other parts.

To overcome the problem of monitoring or measuring low strain levelsvarious forms of strain multipliers have been devised. The purpose ofsuch multipliers is to increase the strain level in an appropriate areaof the structure by a known factor so that the strain level can beconveniently measured. A common type of strain multiplier, sometimesreferred to as a strain amplifier, is illustrated in FIG. 1. In thistype of strain multiplier the sensing gage (strain gage or fatigue lifegage) is bonded to the small center portion of the multiplier, which isin turn attached at ends A, A to'the structure in which strains are tobe measured. Surface strains in the structure are transmitted to themultiplier by virtue of the end attachments (A,A) and thus appear in thecentral portion to which the sensing gage is bonded. The strains presentat the central location of the sensing gage are increased by themultiplier since the strains between ends A,A are concentrated in thereduced center section.

Another known multiplier arrangement which operates on the sameprinciple as that illustrated in FIG. 1, appears in FIGS. 2(a) and 2(b).The reduced center section of this multiplier (see FIG. 2(b)), where thesensing gage is bonded, is produced by thinning (i.e., reducing thethickness) of the multiplier instead of narrowing it.

While strain multipliers of the types illustrated by FIGS. 1, 2(a) and2(b) are currently employed, they suffer from several seriousdeficiencies which greatly limit their usefulness. First, the amount ofstress required to strain the central portion of such multipliers isvery high, and all of this force (stress) must be transmitted though theend attachments. If adhesive bond- 7 ing is used for multiplierattachment, the bonded areas must be quite large and very carefulbonding practice must be followed with high strength adhesives. Thisrequires the multipliers to be physically quite large. Second, themultiplier represents a very substantial reinforcement or stiffeningmember because of its size and force requirement. Accordingly, themultiplier can not be used on thin structures such as aircraft wingpanels without completely altering the stress distribution in thestructure. Third, the multiplier must be unbonded from the structureexcept at the ends and will therefore tend to vibrate under certainforms of structural loads, such as those caused by shock or impact.These vibration forces can be large enough to cause the multiplier tobecome completely unbonded at one or both ends and hence inoperative.

In an effort to overcome the deficiencies present with the multipliersillustrated in FIGS. 1, 2(a) and 2(b), other forms of strainmultipliers, such as that illustrated by FIGS. 3(a) and 3(b) have beendeveloped. In the multiplier shown in FIGS. 3(a) and 3(b) the centralportion of the multiplier is slotted in such a way that the forcerequired to displace the two ends is greatly reduced. A sensing gage isbonded across the slotted area, and the multiplier is attached to thedesired structure by bonding the two ends in the normal manner. Thistype of multiplier has the advantage of providing less reinforcement tothe structure and permitting the physical size of the multiplier to bereduced. The slots, however, create an undesirable nonuniform straindistribution in the sensing gage which is bonded across them. Thisundesirable effect can be reduced somewhat by bonding a thin sheet ofplastic between the slotted multiplier surface and the bottom of thesensing gage. In order to prevent the tendency of the slots to fill upwith adhesive used to install the sensing gage and/or plastic sheet, theslots can be filled with a soft rubberlike material.

It will be seen that the central portion of all the aforementionedmultiplier designs, which is responsible for practically all of theundesirable force requirements and reinforcement effects, is absolutelyessential to permit proper operation of the bonded sensing gage.However, if the slotted portion is removed from FIGS. 3(a) and 3(b), forexample, and the sensing gage is then bonded between the two ends of themultiplier, compressive strains transmitted to the sensing gage wouldsimply cause it to buckle. The slots, therefore, are needed to allowboth tensile and compressive strains to be properly sensed, in additionto providing sufficient strength so that the multiplier may be handledand installed in a practical manner.

The multiplier design of FIGS. 3(a) and 3(b) thus has a deficiency whichis common to other designs, viz., the central portion, bearing thesensing gage, must be unbonded from the surface and is subject to damagefrom shock and vibration, from personnel working on the structure nearthe multiplier installation, etc. Another serious limitation is that thecentral portion of the multiplier is weak and flexible and extremelycareful handling is needed before and during installation to avoidoverstraining and damaging the sensing gage. Since the sensing gage isusually prebonded to the top surface of the multiplier before themultiplier is installed on the structure, handling forces that cause anybending of the multiplier are particularly detrimental. Moreover, theslotted multiplier is a delicate part to machine because of the narrowslots, and slight distortion in the slotted area can alter gage responseenough to degrade accuracy and repeatability. It is also difficult tocompletely fill all of the slots of a slotted multiplier with a softrubber compound without occasional voids or bubbles that contribute toloss of accuracy.

Another limitation of all multiplier designs discussed above lies in thethickness of the central portion and the height at which the bondedsensing gage is consequently operating above the surface of thestructure. This causes serious errors when the structure is fairly thinand strains due to bending stresses are present. As an example, if thestructure is an aircraft wing panel 1/16 inch thich (0.0625 inches), anda multiplier such as that shown in FIG. 3(b) is employed with an overallthickness of approximately one thirty-second inch (0.0312 inch), theerror in sensing bending strains will approach 100 percent. The lowerlimit on possible thickness of the slotted multiplier is set bypractical considerations and the fact that the slotted area itself mustbe thick enough to prevent instability, e.g., buckling when compressivestrains are applied.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a strain multiplier or strain amplifier, and a process formaking same, which may be used with bonded strain gages or fatigue lifegages so as to provide an accurate and predictable multiplication ofstructural surface strain.

Another object of this invention is to provide a strain multiplier whichreduces the required operational forces to substantially the forcerequired to strain the sensing gage which is used with the strainmultiplier.

Still another object of this invention is to provide a strain multiplierin which the entire multiplier sensing gage assembly can be completelybonded to a structure without any free or unbonded portion which couldrespond to unwanted forces.

A further object of this invention is to provide a strain multipliersensing gage assembly in which the sensing gage is extremely close to astructural surface so as to permit accurate response to bending strainsin thin structures.

Yet another object of this invention is to provide a strain multipliersensing gage assembly which is very small and light and can be installedon small structures or in restricted areas.

A still further object of this invention is to provide a strainmultiplier sensing gage assembly which is relatively unaffected by beingbent and handled during installation, and yet which is compliant andformable enough to be installed on moderately curved surfaces.

An even further object of this invention is to provide a strainmultiplier in which the amplified strain level applied to any associatedsensing gage is completely uniform throughout the entire active lengthof such sensing gage thereby resulting in high accuracy and long gagelife.

In accordance with the present invention, a strain multiplying device isprovided having rigid members, which are adapted to be attached to astructural surface, separated by a material of low modulus ofelasticity, which material is attached to the rigid members and adaptedto be attached to the structural surface. The rigid members areseparated from each other by a gap across which a strain responsivedevice can be attached. The strain responsive device is separated fromthe structural surface by the material of low modulus of elasticity.

BRIEF DESCRIPTION OF THE DRAWINGS Other and further objects, advantages,and features of the invention will be apparent to those skilled in theart from the following detailed description thereof, taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a diagrammatic drawing of a known strain multiplier, attachedto a structural surface, with a strain responsive device attached tosaid multiplier;

FIGS. 2(a) and 2(b) are the top and side diagrammatic drawings,respectively, of a sensing gage attached to a known strain multiplierwhich has a center portion of reduced thickness, in which FIG. 2(b)shows the attachment of the multiplier to a strucural surface by meansof adhesive;

FIGS. 3(a) and 3(b) are the top and side diagrammatic drawings,respectively, of a sensing gage attached to a known strain multiplierwhich has a slotted center portion, in which FIG. 3(b) shows theattachment of the multiplier to a structural surface by means ofadhesive;

FIG. 4 is a diagrammatic drawing of the side of a strain multiplier madein accordance with one embodiment of the present invention;

FIGS. 5(a) and 5(b) are the top and side diagrammatic drawings,respectively, of the strain multiplier illustra'ted in FIG. 4 showingthe strain multiplier attached to a structural surface; and

FIG. 6 is a diagrammatic drawing, completely in cross section, of theside of another embodiment of a strain multiplier made in accordancewith the present invention.

It will be understood that the scale of the drawings, and particularlythe vertical scale of FIGS. 4, 5(b) and 6, has been greatly enlarged tofacilitate clarity; FIG. 6 being drawn to a scale which is differentfrom that of FIGS. 4, 5(a), and 5(b). It will also be understood thatfor simplicity the adhesive layer between the sensing gage andmultiplier in FIGS. 2(b), 3(b), 4, 5(b), and 6 has not been shown.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1, 2(a), 2(b), 3(a) and3(b), showingknown strain multipliers, have been described above inconnection with the background of this invention.

Referring to FIG. 4, which illustrates one embodiment of a strainmultiplier made in accordance with the present invention, the multiplierhas two identical rigid members consisting of two end-extenders, ll,usually made of a high-modulus material such as metal, which are spacedapart by a precisely defined gap, 12, having a width defined by linesB-B. This gap is filled with a strong but flexible material having a lowmodulus of elasticity, such as rubber or an elastomeric compound. Asensing gage or strain responsive device, 13, such as a metal wire orfoil sensing grid encapsulated in a matrix of insulating resin of thetype commonly used in the construction of strain gages or fatigue lifegages, is attached, e.g., adhesively bonded, to the underside ofend-extenders l1. Sensing gages of this type have a definite length ofthe grid, usually called the gage length,

located near the center portion of the gage matrix. This is thestrain-sensitive section of the sensing gage and the gage matrix is sopositioned in the multiplier that the gage length dimension is the sameas the length of the gap, 12, which is indicated as dimension B-B in thedrawings.

Leadwires, 15, extend from sensing gage l3, and are shown passingthrough a hole in one of the endextender pieces for eventual connectionto an appropriate and conventional electrical measuring instrument. Theentire area, 17, between the mounting-foot portions, 18, ofend-extenders 11 is filled with either the same material or a similarmaterial used to fill gap, 12.

The strain multiplier-sensing gage assembly shown in FIG. 4 is thereforean assembly of parts firmly bonded toghether, presenting smooth,unbroken surfaces on all sides. Such an assembly typically has adimension of less than one inch in length, and a thickness ofoneeightieth inch (0.0125 inch). This thickness would be made up, forexample, of a sensing gage thickness of about 0.002 inch, an elastomerthickness of about 0.005 inch above the gage (in the gap area), and anelastomer thickness of about 0.005 inch below the gage. Because of thedesign of the parts involved, the corresponding dimensions of theend-extenders are then defined as about 0.005 inch thick in the centerportions, and about 0.0125 inch thick in the mounting foot portions.Typically, the width of such a multiplier assembly is Vs to A inch,depending on the specific sensing gage employed.

Since the multiplier assembly is very thin and since the center portionconsists mainly of a rubberlike material, the multiplier' assembly willobviously be very flexible. Preferably, the metal end-extenders have ahigh modulus of elasticty, but may have a very low tensile strength andconsequently, a great deal of ductility. This permits the entiremultiplier assembly to conform to the surface of a curved structure withvery little difficulty.

It will also be seen that in this multiplier design the relativelydelicate sensing gage is located almost exactly in the center, withessentially an equal thickness of elastomer or rubber compound on bohthits top and bottom. This places the plane of the sensing gage on theneutral axis of the center portion of the multiplier when bending forcesare applied, and therefore very little stress occurs in the sensing gagedue to bending. In fact, the multiplier assembly of FIG. 4 may be bentmore than 90 through the middle without damage to the sensing gage ormultiplier. In contrast to this, all of the previous multipliersillustrated (FIGS. 1, 2(a), 2(b), 3(a) and 3(b) have the sensing gagelocated on the outer portion at the center of the multipliers. Just afew degrees of bending applied to these multipliers will createsufficient forces to crack the sensing gage and render it inoperative.These previous designs are consequently delicate as well asnoncomformable to curved surfaces.

The end-extenders 11 and the mounting foot portion 18 of the multipliercan be made of the same material or different materials attached to eachother by some suitable means such as spot welding. Rigid, materialshaving a high modulus of elasticity are used for those portions of themultiplier. Typically, such multipliers are made of a metal or an alloy(e.g. aluminum alloy, stainless steel, etc.) which can be machined orfabricated into the desired shape. However, any other rigid materialhaving a high modulus of elasticity can be used, such as glass, ceramic,plastic and the like. Advantageously, the multiplier is made of materialsimilar to the structure on which it is to be bonded since this providesa considerable amount of temperature compensation. In some instances,the most effective temperature compensation may be achieved by choosinga multiplier material which differs by a known amount from the thermalexpansion coefficient of the structural material.

The material present in gap 12 and area 17 can be the same or different.Rubber like material which has a low modulus of elasticity is preferred.Examples of such material include polysulfides, silicone, rubber (e.g.,Silastoseal B), natural rubber, polyurethanes, and the like. It isessential that the rubber like material used is capable of being firmlybonded to the structure to be tested, to the multiplier and to thesensing device. Deformable material which can be used might have, forexample, a modulus of elasticity of approximately 100 psi, a tensilestrength of about 250 psi and an elongation capability of about 350percent.

The same or a different adhesive can be used for bonding the multiplier,the material of low modulus of elasticity and the sensing device.Typically, epoxy cements and similar types of materials, e.g., Bakelitecements, Duco household cement, Glyptal, shellac, polyimides areemployed. Specific epoxy cements contemplated include EPON 6 and thoseidentified in U. S. Letters Patent No. 3,372,219. It will be understood,however, that the mounting foot portions can be soldered or brazed tothe structure to be tested. The mounting foot portions can also be spotwelded to the structure, if desired.

Referring to FIGS. 5(a) and 5(b), these figures shown the sensing gagemultiplier assembly of FIG. 4 adhesively bonded on a structural surface,20, with an appropriate bonding agent, 21. FIG. 5(a) is a top view ofthe installation, while FIG. 5(b) is a side view. As mentioned above thebonding agent which is used must be rigid and strong, with high adhesionto all of the materials it contacts. The preferred adhesive is of theepoxy resin type, compounded for rigid (high modulus) properties, andpreferably moderate or room temperature curing cycles.

It will be observed that bonding agent 21 fills the entire space betweenthe multiplier assembly and the structural surface 20, and thereforeattaches both the mounting feet, 18, and the lower surface of theelastomer material, 17, to the structure. The actual thickness of theadhesive is not critical, but is preferably in the range of about 0.0001inch to about 0.001 inch.

The operation of the multiplier-sensing gage assembly, illustrated byFIGS. 4, 5(a) and 5(b), is described below, reference lines AA in FIGS.4, 5(a) and 5(b) are drawn through the center points of the rigidmounting feet and the distance between these lines can be considered thegage length of the multiplier assembly. Strains developed in thestructural surface between lines A-A will cause a correspondingdisplacement of the mounting feet since they are rigidly attached to thesurface. The displacement of the mounting feet due to structural strainwill also appear betweeen the two ends of the end-extenders in thecenter gap region 12. The sensing gage, 13, is rigidly bonded to thebottom surface of the end-extenders, 11, with the strainresponsive gridof the gage located between the ends of the gap. The displacement of theends of the gap, between lines B-B, is therefore transmitted to thesensing grid of gage 13.

It will be seen that the strain transmitted to the sensing grid isincreased over the strain level on the structural surface between linesB-B by the ratio of the distance between lines AA (multipliergage-length) to the distance between lines B-B (sensing gagegagelength). This ratio is therefore the theoretical multiplicationfactor (MF of the multiplier assembly.

Without the presence of the elastomer section, 17, the action of themultiplier would fail, because while a tensile strain between lines BBwould properly act on the sensing grid of the gage (by stretching it), acompressive strain would simply cause a buckling deformation of the gagein the gap area, and the gage would therefore not sense the compressivestrain. Buckling would occur in any case such as this, because the gagelength of the sensing gage (the distance between lines B'-B) is verylarge compared to the thickness of the sensing gage, 13. The ratio ofthese two quantities normally ranges between about 30 and about 100.This ratio of length to thickness for elements in compression iscompletely unstable. Since the actual displacements involved in a casesuch as this are very small, the compressive buckling in the gage wouldappear simply as a slight bowing of the sensing gage in the gap area inan upward or downward direction. Compressive buckling would also occurin the end-extenders, 11, between the mounting feet, because theseelements are also thin compared to their length and would be completelyunstable under compression.

Thus, the presence of the elastomer section, 17, restores compressivestability to the end-extenders and sensing gage, and completely preventsbuckling. The importance of the elastomer section may be explained inthe following manner. Buckling in thin plates under compression causesthe plates to bow and large enddisplacements of the plates will occurwith very little force required because the buckling is due to bendingstress in the plate rather than axial or plane stress. Because bucklingproduces such low forces in the direction of bending, only small forcesare required to prevent buckling. In the embodiment illustrated by FIGS.4, (a) and 5(b), the entire lower surface of the endextenders, 11, andthe sensing gage, 13, is bonded at every point to the structuralsurface, 20, through the thin elastomer section 17. Even though thiselastomer has a very low elastic modulus, it provides a very largerestraint for normal (i.e., forces perpendicular to the surface) betweenthe bonded surfaces for extremely small displacements. The effectiverestraint provided by'the elastomer in the normal direction is actuallymany times greater than that required to prevent buckling in the sensinggage and end-extenders, and as a result, both compressive and tensilestrains are faithfully transmitted to the sensing grid.

The action of the elastomer in preventing buckling, as described above,would be provided equally well by a rigid compound. A rigid compound,however, would have a sufficiently high shear modulus (or shearrigidity) to prevent the displacements between lines AA from appearingin the center gap between lines 13-8. The multiplication ratio wouldfall to approximately 1 and no multiplier effect would be present.

For a multiplier-sensing gage assembly, such as that of FIGS. 4, 5(a)and 5(b), the axial force required to develop tensile or compressivestrains of 5,000 microstrain in the sensing gage is about 6 pounds. Theforce required to develop this strain level in the elastomer at thelower surface of the sensing gage would be a small fraction of onepound. Therefore, the presence of the elastomer does not appreciablyaffect the efficiency of the multiplier. The 6 pound force on thesensing gage must be provided by the adhesive bond between themultiplier mounting feet and the strucutural surface. Since a typicaldimension for the mounting feet is 0.2 inches long by 0.2 inches wide,this provides a bonding area of 0.04 square inches. A 6 pound forcewould accordingly cause a shear stress in the adhesvie under themounting feet of 125 pounds per square inch. Since common rigidstructural adhesives easily demonstrate shear strengths in excess of2,000 psi, a large margin of safety is provided.

Since the 6 pound actuating load for the sensing gage must be carried bythe end-extenders, 11, a small amount of axial deformation will occur inthe endextenders and cause the actual multiplication factor MF to beslightly lower than the theoretical multiplication factor MF Using thedimensions set forth above for end-extenders made of stainless steelhaving a 30 X 10 elastic modulus the reduction in the multiplicationfactor would be about 8 percent, so that MF =0.92 MFT. .This reductioncan be eliminated to any extent desired by simply iricreasing the wdithand/or thickness of the end-extenders, but in the interest of very smallmultiplier thickness it is usually more convenient to compensate byincreasing MF slightly.

Comparing the operating constants for the multiplier design of FIGS. 4,5(a) and 5(b) with the known multipliers previously discussed, overallthickness is reduced by a factor between about 3 and about 20; Thesensing grid plane is closer to the structural surface by a factor of atleast 5; and the actuating force for the multiplier is lower by a factorof at least (in the case of the designs of FIGS. 1, 2(a) and 2(b) and bya factor of at least 3 (in the case of the design of FIGS. 3(a) and3(b)). The large reduction in operating force greatly reducesreinforcement effects in the structure under test and renders bondingfailures in service unlikely. The resulting smaller allowable mountingfeet dimensions more precisely define exact multiplier gage length andimprove repeatability of the multiplier-sensing gage assemblies. Manyother points of improved performance exist, but perhaps the one ofgreatest importance is that the multiplier assembly constructedaccording to the present invention is a completely bonded and unitarypart of the structural surface wtihout an unbonded free component. Thismultiplier-sensing gage assembly is therefore far less likely to sufferdamage or produce extraneous error signals under conditions of shock orvibration.

A multiplier was desinged and built in accordance with the presentinvention, based on the multiplier of FIG. 4. When allowance was madefor stiffness of the end-extenders, the corrected MF was 5.27. Theacutal multiplication factor MF was measured as 5.06, or a reduction inthe multiplication factor of 4 percent. No effort was made to correctfor second-order effects, such as shear-strain in the glue line of themounting feet. The tested multiplier/sensing gage assembly demonstratedperfect linearity intension and compression, and was cycled over1,000,000 times at a multiplied strain level of $2,500 microstrain tocheck durability. The multiplier/sensing gage assembly was in perfectcondition at the end of this test.

It will be understood-that many practical variations in design of themultiplier or multiplier/sensing gage assembly may be made withoutdeparting from the spirit of this invention. Referring to FIG. 4, forexample, the sensing gage could be mounted on the top surface of theend-extenders and covered with a proper thickness of elastomer. Thiswould place the sensing grid slightly further from the structuralsurface, but would lower production cost and would be very suitable forthicker structures. An embodiment of this type is shown in FIG. 6. Inthe embodiment illustrated by FIG. 6, endextenders 25 and mounting feet26 are separate parts which are attached or bonded together by suitablemeans, such as spot welding. It will be understood that, if desired,these portions of the multiplier could be unitary, similar to themultiplier design of FIG. 4. Like the multiplier design of FIG. 4, theend-extenders 25 are separated by a gapand both this gap and the cavitybeneath the end-extenders are filled with a deformable material 28. Thesensing gage 29 is mounted on top of the end-extenders in thisembodiment, and the grid portion is centered, as in FIG. 4, with respectto the gap between the end-extenders. An extension of the gage carriesthe attachment terminals 30. Deformable material 31, which can be thesame as or different than deformable material 28, covers and is attachedto endextenders 25 and sensing gage 29. Attachment terminals 30 on thesensing gage of the multiplier/sensing gage assembly of FIG. 6 aredirectly accessible for wiring to a suitable readout instrument.

In any of the embodiments of the present invention it will be seen thata strain gage and fatigue life gage can be bonded together on themultiplier for simultaneous readout of both strain level and fatiguedamage. Moreover, these gages can be so connected to the readoutinstrument as to provide automatic correction for static strain level inthe fatigue life gage reading. If so desired, another strain gage couldalso be bonded to the lower surface of the elastomer section (17 in FIG.4 and 28 in FIG. 6) so that it would be directly bonded to thestructural surface when the multiplier assembly is installed. Thereading of such a strain gage compared to the reading of the upper(multiplier) strain gage would then yield the exact multiplicationfactor for that multiplier assembly in the installed condition. It willbe understood that, if desired, multiple strain responsive devices canbe attached across the gap formed by the rigid members of theamplification devices of the present invention.

It will also be understood that in certain cases the sensing gage couldbe attached directly to the mounting feet, such that a gap is defined bythe mounting feet.

Since the multipliers made in accordance with the present invention canbe bowed, and thus follow the contours of a surface, wihtout substantialadverse effect, there is no theoretical limit, at least, to the use ofsuch multipliers on curved structures. Since the present multiplierdesign allows any desireable gage length to be chosen for the sensinggage, and any desirable gage length for the multiplier, there is notheoretical limit on the multiplication factor which can be obtained.

From the foregoing, it will be seen that this invention is well adaptedto obtain all of the ends and objects hereinabove set forth, togetherwith other advantages which are obvious and which are inherent therein.

Obviously, many other modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof.

What is claimed is:

l. A strain multiplier which comprises deformable material having a lowmodulus of elasticity bonded to at least two rigid members and a strainresponsive device to form a unitary structure, wherein each of the rigidmembers is composed of a material of high modulus of elasticity andcomprises a mounting portion and extension portion which are at rightangles to each other so maintained that the ends of said extensionportions are directed toward each other and are spaced slightly apartwith the strain responsive device attached to the rigid members acrossthe space between said extension portions, the said deformable materialbeing bonded to the surface of the strain multiplier between themounting portion of each rigid member.

2. Strain multiplier comprising at least two noncontiguous rigid membersadapted to being bonded to a structural surface throughout their entirelength, each of said members having a mounting portion for attachment toa structural surface and a cantilever extension portion attached to themounting portion such that the cantilever extension portions aredirected toward each other and spaced apart to form a gap between thecantilever extension portions across which at least one strainresponsive device is attached, wherein said strain multiplier hasdeformable material present in the gap between said extension portionsand bonded to the bottom surface of the rigid members and each strainresponsive device.

3. The method of forming a strain multiplier which comprises bondingdeformable material having a low modulus of elasticity to at least tworigid members and a strain responsive device to form a unitarystructure, wherein each of the rigid members is composed of a materialof high modulus of elasticity and comprises a mounting portion andextension portion which are at right angles to each other so maintainedthat the ends of said extension portions are directed toward each otherand are spaced slightly apart with the strain responsive device attachedto the rigid members across the space between said extension portions,the said deformable material being bonded to the surface of the strainmultiplier between the mounting portion of each rigid member.

1. A strain multiplier which comprises deformable material having a lowmodulus of elasticity bonded to at least two rigid members and a strainresponsive device to form a unitary structure, wherein each of the rigidmembers is composed of a material of high modulus of elasticity andcomprises a mounting portion and extension portion which are at rightangles to each other so maintained that the ends of said extensionportions are directed toward each other and are spaced slightly apartwith the strain responsive device attached to the rigid members acrossthe space between said extension portions, the said deformable materialbeing bonded to the surface of the strain multiplier between themounting portion of each rigid member.
 2. Strain multiplier comprisingat least two non-contiguous rigid members adapted to being bonded to astructural surface throughout their entire length, each of said membershaving a mounting portion for attachment to a structural surface and acantilever extension portion attached to the mounting portion such thatthe cantilever extension portions are directed toward each other andspaced apart to form a gap between the cantilever extension portionsacross which at least one strain responsive device is attached, whereinsaid strain multiplier has deformable material present in the gapbetween said extension portions and bonded to the bottom surface of therigid members and each strain responsive device.
 3. The method offorming a strain multiplier which comprises bonding deformable materialhaving a low modulus of elasticity to at least two rigid members and astrain responsive device to form a unitary structure, wherein each ofthe rigid members is composed of a material of high modulus ofelasticity and comprises a mounting portion and extension portion whichare at right angles to each other so maintained that the ends of saidextension portions are directed toward each other and are spacedslightly apart with the strain responsive device attached to the rigidmembers across the space between said extension portions, the saiddeformable material being bonded to the surface of the strain multiplierbetween the mounting portion of each rigid member.